One type of conventional information recording medium is a phase-change type information recording medium that utilizes a phenomenon in which a recording layer (phase-change material layer) included in the medium undergoes a phase change. Among the phase-change type information recording media, there is a type of information recording medium that utilizes a laser beam to record, erase, rewrite or reproduce information optically. This type of information recording medium has a recording layer made of a phase change material that changes its state between, for example, a crystalline phase and an amorphous phase by heat generated by irradiation with a laser beam. A difference in reflectance between the crystalline phase and the amorphous phase is detected and read out as information.
Among the phase-change type information recording media, a rewritable information recording medium is one in which recorded information can be erased or rewritten. This rewritable information recording medium includes a recording layer whose initial state is usually in the crystalline phase. When recording information, the recording layer is melted by irradiation with a laser beam of high power (recording power) and cooled rapidly, so that a portion irradiated with the laser beam is changed to the amorphous phase. On the other hand, when erasing information, the recording layer is heated by irradiation with a laser beam of lower power (erasing power) than that applied during recording and cooled slowly, so that a portion irradiated with the laser beam is changed to the crystalline phase. Therefore, it is possible to record new information or rewrite information on the rewritable information recording medium while erasing recorded information by irradiating the recording layer with a laser beam whose power is modulated between a high power level and a lower power level. In order to change the recording layer into the crystalline phase, the recording layer needs to be maintained for a given period of time (crystallization time) at a temperature (crystallization temperature) for allowing the recording layer to change its phase into the crystalline phase. A time required for changing the recording layer into the crystalline phase becomes shorter when its crystallization time is shorter, and thus information can be erased or rewritten in a shorter time, that is, at a higher speed.
In addition, among the phase-change type information recording media, a write-once information recording medium is one in which information can be recorded only once and the recorded information cannot be erased or rewritten. The write-once information recording medium includes a recording layer whose initial state is usually in the amorphous phase. When recording information, the recording layer is heated by irradiation with a laser beam of high power (recording power) and is then cooled slowly, so that a portion irradiated with the laser beam is changed into the crystalline phase.
In recent years, various techniques have been introduced to increase the recording capacity of optical information recording media. For example, a technique for achieving high density recording has been introduced in which a blue-violet laser having a short wavelength is used, a thin substrate is provided on the side from which a laser beam is incident, and an objective lens having a large numerical aperture (NA) is used so as to reduce the spot diameter of the laser beam.
Another technique has been introduced in which an optical information recording medium provided with two information layers is used to double the recording capacity and information is recorded/reproduced on/from the two information layers using a laser beam that is incident from the side of one of the two information layers (see, for example, JP 2000-36130 A, pages 2 to 11, FIG. 2). In this two-layer optical information recording medium, a laser beam that has passed through one information layer (hereinafter, a “first information layer”) disposed closer to the incident side of the laser beam is used to record/reproduce information on/from another information layer (hereinafter, a “second information layer”) disposed farther away from the laser beam incident side. Therefore, in the first information layer, a recording layer and a reflective layer provided for allowing the recording layer to absorb light efficiently and for increasing the reflectance of the first information layer must be extremely thin. In addition, a transmittance adjusting layer having a high refractive index needs to be disposed to increase the transmittance further by optical interference. Conventionally, TiO2, which is an oxide of titanium (Ti), is used for the transmittance adjusting layer to increase the transmittance of the first information layer (see, for example, WO 03/025922, pages 10 to 20, FIG. 1).
However, a TiO2 layer conventionally used as a transmittance adjusting layer has a problem in that when the TiO2 layer is formed by sputtering, which is a commonly known film forming method, the thickness of the film formed per unit time (hereinafter referred to as a “film formation rate”) is small and it takes a long time to obtain a desired thickness (that is, the film formation rate is low). Furthermore, the film formation rate of the TiO2 layer varies within a given range of values because it is dependent on the degree of vacuum in the film forming chamber. Therefore, the TiO2 layer has another problem in that it is difficult to maintain the thickness of the TiO2 layer within a desired thickness range particularly during mass production thereof. A multi-layer optical information recording medium having three or more information layers, which is expected to be commercialized in the future, has an increased number of information layers on the laser beam incident side. Accordingly, the number of TiO2 layers serving as transmittance adjusting layers also increases, which may make this problem more serious.